The invention relates to an antenna device including a microstrip antenna.
A microstrip antenna comprises a dielectric member, a conductor member mounted on the dielectric member, and a ground conductor member mounted on the opposite surface of the dielectric member from the conductor member, and represents an antenna which utilizes a radiation loss of an open plane resonance circuit. Attention is now being directed to such microstrip antenna due to its low profile, reduced weight, compactness and ease of manufacture.
FIG. 3 illustrates one form of conventional microstrip antenna device. As shown, the device comprises a dielectric plate 110, one surface of which is applied with a radiating conductor sheet 120 formed by a copper foil while the opposite surface is applied with a ground conductor sheet 130 again formed by a copper foil. The device includes a feeder 121 in which a small hole 111 is formed extending through the dielectric plate 110, conductor sheet 120 and ground sheet 130. A connector 140, or more precisely, an external conductor associated therewith, is soldered to the ground sheet 130, and an internal conductor or core of the connector 140 is connected to a gold plated wire 141 which is soldered to the feeder portion of the radiating conductor sheet 120. The hole 111 is filled with an insulating material, not shown, thereby insulating the wire 141 from the ground sheet 130. The connector 140 is connected with a coaxial cable 150 which is in turn connected to a high frequency amplifier of a receiver unit.
In a receiver unit which is herein understood to be a circuit portion extending from the antenna feeder to an output device such as a loudspeaker, a cathode ray tube or the like, disturbances caused by external noises which are applied in a region between the antenna feeder and a first stage amplifier or the high frequency amplifier of the receiver unit have a great influence upon the noise figure of the receiving unit because they are amplified by the high frequency amplifier and every amplifier in an electrical path subsequent thereto. Obviously, the greater the length between the feeder and the initial amplifier, the greater the influence of these disturbances.
By way of example, when an antenna device such as shown in FIG. 3 is assembled into an outside panel of a vehicle with a coaxial cable 150 being used to provide a connection between the connector 140 and an input terminal of a receiver which is disposed within an instrument panel of the vehicle, a substantial length is required of the cable 150 and a received signal from the feeder 121 experiences an increased attenuation during its transmission through the connector 140 and through the cable 150 due to attenuation coefficients of the connector 140 and the coaxial cable 150. On the other hand, the coaxial cable is subject to disturbances of external noises over its full region including the connector 140, and this considerably degrades the noise figure of the receiving unit, i.e. between the antenna feeder 121 and an output source such as a loudspeaker or cathode ray tube unit.